liminfoSExtractor Reference
Free reference guide: SExtractor Reference
About SExtractor Reference
The SExtractor Reference is a focused quick-reference guide for astronomers and data scientists working with SExtractor (Source Extractor), the widely used astronomical source detection and photometry software. It covers the core configuration parameters including DETECT_THRESH, DETECT_MINAREA, ANALYSIS_THRESH, convolution filter selection (FILTER_NAME), and deblending controls (DEBLEND_NTHRESH, DEBLEND_MINCONT) with their default values and practical tuning advice.
This reference provides detailed coverage of photometry configuration including MAG_ZEROPOINT calibration, PIXEL_SCALE setting, detector GAIN for Poisson noise calculation, SEEING_FWHM for PSF characterization, and aperture photometry diameters (PHOT_APERTURES). It also explains the key output parameters: MAG_AUTO (Kron elliptical aperture magnitude), FLUX_AUTO, CLASS_STAR star/galaxy classifier (0-1 index), ELLIPTICITY, and the FLAGS bitmap for quality filtering.
Advanced topics include background estimation tuning (BACK_SIZE mesh, BACK_FILTERSIZE, BACKPHOTO_TYPE global vs. local), dual-image mode for multi-band photometry where detection and measurement images are separated, weight/RMS/variance map usage for non-uniform noise handling, diagnostic check images (BACKGROUND, SEGMENTATION, APERTURES), and output catalog format options including FITS_LDAC for downstream tools like SCAMP and SWarp.
Key Features
- Detection parameter reference: DETECT_THRESH sigma thresholds, DETECT_MINAREA pixel limits, and convolution filter options
- Deblending configuration: DEBLEND_NTHRESH sub-threshold count and DEBLEND_MINCONT contrast values for overlapping source separation
- Background estimation: BACK_SIZE mesh sizing, BACK_FILTERSIZE smoothing, and GLOBAL vs. LOCAL photometry modes
- Photometry setup: MAG_ZEROPOINT calibration, GAIN, PIXEL_SCALE, SEEING_FWHM, and multi-aperture PHOT_APERTURES
- Output parameters: MAG_AUTO Kron magnitude, CLASS_STAR classification, ELLIPTICITY, FLAGS quality bitmap
- Dual-image mode syntax for multi-band photometry with separate detection and measurement FITS files
- Weight map integration (MAP_WEIGHT, MAP_RMS, MAP_VAR) for handling non-uniform noise across the field
- Check image generation for diagnostics: background maps, segmentation maps, and aperture overlays
Frequently Asked Questions
What is SExtractor and what does it do?
SExtractor (Source Extractor) is an astronomical software tool that automatically detects and measures sources (stars, galaxies, and other objects) in FITS images. It produces catalogs with positions, photometric measurements (magnitudes, fluxes), morphological parameters (ellipticity, FWHM), and star/galaxy classification. It is one of the most widely used tools in observational astronomy.
How do I tune DETECT_THRESH for faint source detection?
DETECT_THRESH sets the detection threshold in units of background sigma. The default 1.5 sigma provides a good balance. Lowering it (e.g., 1.0) detects fainter sources but increases spurious detections. Raising it (e.g., 3.0) gives cleaner catalogs but misses faint objects. Combine with DETECT_MINAREA (minimum connected pixels) to filter noise.
What is dual-image mode and when should I use it?
Dual-image mode uses the syntax: sex detect.fits,measure.fits -c config.sex. The first image is used only for source detection (positions), and the second for photometric measurement. This is essential for multi-band photometry, ensuring consistent source positions across all wavelength bands while measuring fluxes in each band separately.
How does MAG_AUTO (Kron magnitude) work?
MAG_AUTO uses an adaptive elliptical aperture based on the Kron radius, which scales to capture a consistent fraction of total flux regardless of source size. The aperture is set by PHOT_AUTOPARAMS (default 2.5, 3.5), where the first value is the Kron factor and the second is the minimum radius. It is generally the best total magnitude estimate for galaxies.
How do I interpret the CLASS_STAR parameter?
CLASS_STAR ranges from 0.0 (galaxy) to 1.0 (star). Values above 0.8 indicate high-confidence point sources. Its accuracy depends heavily on the SEEING_FWHM setting matching actual observing conditions. It is most reliable for bright, well-detected sources and becomes unreliable near the detection limit or in crowded fields.
What do the FLAGS values mean?
FLAGS is a bitmap where bit 1 = neighbor overlap, bit 2 = deblended object, bit 4 = saturated pixels, bit 8 = image boundary truncation, bit 16 = incomplete aperture. FLAGS=0 means a clean, uncontaminated detection. For photometric catalogs, filtering on FLAGS=0 or FLAGS < 4 is common practice to ensure reliable measurements.
How do I configure background estimation for crowded fields?
For crowded fields, reduce BACK_SIZE (e.g., 32 instead of 64) to track finer background variations. Set BACKPHOTO_TYPE to LOCAL instead of GLOBAL so each source uses its local background. Increase BACK_FILTERSIZE for additional smoothing if the background map shows artifacts from bright sources.
What output catalog formats does SExtractor support?
SExtractor supports FITS_LDAC (recommended for SCAMP/SWarp pipeline), FITS_1.0 (standard FITS binary table), ASCII_HEAD (text with column header), and ASCII (plain text). The output columns are controlled by the .param file specified with PARAMETERS_NAME, where you list desired measurements like NUMBER, X_IMAGE, Y_IMAGE, MAG_AUTO, FLUX_AUTO, CLASS_STAR.